The method of artifically lifting fluid from underground formations used in the vast majority of oil wells has changed little since the earliest history of the petroleum industry. The conventional method uses a surface mounted mechanical system generally referred to as a pump-jack or walking beam. The mechanical drive system imparts a vertical reciprocating motion to the sucker rod string that is connected to the subsurface pump at the formation.
Considerable refinement of the components has occurred through the years including improved metallurgy that enables them to operate reliably. In spite of these improvements, the pump-jack system has certain inherent limitations which has inspired others to attempt devising better methods of artificial lift. To the best of my knowledge none of these alternate systems has achieved much commercial success. Some of the limitations of the conventional pump-jack, sucker rod systems are as follows:
1. High energy requirements. This is a result of using a counterweight on one of the rotary members of the pump-jack to counterbalance the weight of the sucker rod string that results in considerable unbalanced forces and thus energy to reciprocate the sucker rod string. PA1 2. High stresses in sucker rod string. The diameter of the sucker rods selected in any installation is relatively small to minimize the unbalanced forces. This results in sucker rod string stresses as high as 28,000 pounds per square inch in many instances. PA1 3. Oscillation and harmonics in sucker rod string. This results from the high stresses used and the high speed reversal of the string due to the simple harmonic motion imparted by the pump-jack. The hgh stress reversals often cause the string to part and require the sucker rod string and subsurface pump to be pulled using a fishing tool. PA1 4. Size and weight of pump-jack. The pump-jack requires expensive foundations thus limiting where they can be installed due to visual pollution, value of surface real estate, irrigation of surface crops and vandalism. Their size also limits how close together the units can be installed. PA1 5. Fixed pumping rate. In order to reduce the fixed production rate of a conventional pump-jack, the pump-jack is periodically stopped. This is often necessary in a stripper well where only a limited amount of production fluid remains in the well. This stopping time can often be as much as 8 or more hours out of 24. It causes the settling of sand and the like in the subsurface pump. Pulling the subsurface pump for cleaning adds to the cost of operating the well. PA1 6. Downstroke is the same speed as upstroke. The speed of the downstroke is limited by the viscosity of the fluid in the formation. This limits the production rates possible.